Development of high brightness light emitting diodes (HB-LED) in recent years has created a demand for a new class of power sources—LED drivers. The present applications of HB-LEDs include, but are not limited to: traffic signals, decorative lighting, automotive lighting (head and tail lights), etc. The LED driver is expected to supply a constant current with low ripple, and to operate from a wide range of input voltages. Many of these applications do not require galvanic isolation of the LEDs from the input. However, many applications require the driver to step its output voltage either up or down depending on the input voltage. Some applications, like automobiles, require the LED driver to meet stringent conducted and radiated EMI standards. Cascaded DC-DC converter topologies (Cuk, SEPIC) have been considered for this applications in the industry due to their low input EMI and the ability to step the input voltage either up or down.
Another example of cascaded converter topologies is a class of so-called “quadratic” converters that can operate with a wide dynamic range of input voltages and achieve large step-down ratios. These topologies are particularly useful for powering low-voltage devices directly from the 110/220VAC line. Cascaded converters having a buck output stage are the most desirable kind for driving HB LEDs due to the ease of controlling their output current and implementation of PWM dimming. However, conventional control methods when applied to cascaded converter topologies suffer start-up and transient performance problems due to lack of control over the input stage current.
Thus, an improved method and apparatus for controlling the output current of cascaded converters having a buck output stage is needed to achieve rugged performance, stability and fast PWM dimming control of the output current.